Core alloy for indirectly heated cathodes



Jan. 22, 1957 E- G WIDEl-l- 2,778,970

CRE ALLOY FOR INDIRECTLY HEATED CATHODES Filed June 74, 1952 v Q) o /o 0 /00 INVENTOR. 7600 l Ev/L5. I/WDELL l a l f A 7 l f y v ATTORNEY United States Patent CORE ALLOY FOR INDIRECTLY HEATED CATHODES Emil Gideon Widell, Bloomfield, N. J., assignor to Radio Corporation of America, a corporation of Delaware j Application l.inne 7, 1952, Serial No. @2,248

1 Claim. (Cl. 313-337) l rhis invention relates to alloys and more particularly to an improved core alloy for indirectly heated or sleeve type cathodes.

An indirectly heated cathode for electron tubes comprises a metal sleeve core having an electron emitting coating on its outer surface such as the oxides of barium` and strontium, for example, and a heater within the sleeve for heating the sleeve and coating to a temperature at which the coating becomes electron emissive.V The heater is usually in the form of a wire that becomes responsive in heat to a suitable voltage energization.

The sleeve core of an indirectly heated cathode has heretofore comprised an alloy including nickel and certain reducing materials for reducing oxides in the emitting coating requiredv for emission. Several problems have been presented in respect of the composition of the sleeve core alloy.

One of these problems concerns the amount and kindV of the reducing materials used. Reducing materials commonly used comprise one or more of carbon, silicon and magnesium.

Another problem that applicant has found in relation to the composition-of the sleeve core alloy, involves Aundesirable heat losses from the cathode by conduction to supporting elements.

The heat losses resulting from .the area of engagement l between the sleeve and va spacer plate increases with increased heat conduction of the sleeve. Thus when the sleeveis characterized by high heat conduction, heat from intermediate and hotterportions of the sleeve isl *icc Another object is to provide an improved alloy for the" core of an indirectly heated cathode having adequate amounts of redncingsubstances therein for good emission terface, or peeling of the coating from the core.`

A further purpose is to provide an alloy for` an indirectly heated cathode core containing cobalt in an amount for improved efficiency of the cathode and for shielding the emitting coating thereon from magnetic heater effects, and `for preserving goed Workability of the alloy.

Applicant has found that magnesium does not form an objectionable amount of interface. such as results from theus'e of silicon nor does it contribute as much to a hardening of the alloy as carbon,l while it possesses desirable reducing properties.v In addition, magnesium has thedesirable characteristic of chemically reacting with sulphur or sulphur compounds in the nickel and cobalt employed' in the alloy.' This permits use of a reduced amount of manganese and in fact makes it feasible to eliminate completely the use of manganese in the alloy.

y The amount of magnesium is appreciably less than that i is present in the alloy an amount from to 50% byI energized by the maximum voltage tolerated by itsy insulatlion and no further heat increase is possible.

A further problem concerns magnetic eects from the heater. Indirectly heated or sleeve type cathodes have heaters that are usually energized by commercial alternating current sources of cycle frequency. This gives rise to cyclically varying magnetic fields that, unless fully neutralized or shielded, extend through the sleeve and adversely affect emission. No successful attempt has been made, so far as applicant is aware, to rely on the sleeve core as a magnetic shield. This is probably because sleeve cores heretofore used have been made of materials tha-t become paramagnetic at the operating temperatures of the cathodes.

Accordingly, it is an object of the invention to provide an improved indirectly heated cathode.

An important purpose of the invention is to provide an .alloy for the core of a sleeve type cathode that effectively overcomes the difculties aforementioned.

Another object is to provide an alloy for the core of a sleeve type cathode including magnesium and cobalt, for improved operation of the cathode in an electron tube.

good results are obtained when cobalt is present in an` amount from 30 to 60% by weight.

From the foregoing, it will be noted that when cobalt Where directly heated cathodes are concerned, the core' is customarily formed to desired shape by drawing through dies. A higher degree of workability of the alloy is desirable when forming to sleeve shape, than when drawing through dies. One reason for this is that more material is involved in a processing operation for providing a sleeve than in a drawing operation.

According to the invention, the amount of carbon is fixed at from 0.04 to 0.15% of the alloy. When this amount of carbon is used with up to 50% cobalt, the alloy possesses desired workability for forming to sleeve shape.

While this relatively small amount of carbon in relation .to prior art tolerances involves a sacrifice in the amount of reducing material in the alloy for best results in a cathode, the carbon is supplemented according to the invention by from 0.04 to 0.10% silicon and 0.04 to- 0.07% magnesium. Silicon in this range is not objectionable from the standpoint of interface formation. While magnesium is a fugitive material, this characteristic is not objectionable when the alloy is used asv the core of an indirectly'h'eated cathode, which is operated' at appreciably lower temperatures than directly heated` cathodes.

It is 'apparent fromy the foregoing therefore that delinite coaction'sv exist between the cobalt and the reducing materials of the alloy for use of the alloy as the core of an indirectly heated cathode. carbon, silicon, Amagnesium and cobalt are correlated to secure an alloy having improved workability, good reduclf cobalt is present in a- Thus, the amounts of' ns..p.r.9prties,..10w. .thermal conductivity andA good, mas; netic shielding properties;

Therefore, according to the invention, a novel and advantageous alloyfora cathode sleeve is providedcontainingby Weight fron1 0.04 to..f0.10%silicon, 0.04.10 0.07%-magnesium, 0.04'to 0.15% carbon, 30 to 50% cobaltand the remainder nickel. Manganese may valso beincluded in an .amount about 0.05% by weight ofthe alloy, although it may be `completely eliminated Without notable adverse effects, vdue to similarity thereto in function'. olfA the ,magnesiurrrin' reactingwith sulphur compounds, as described heretofore.

Further objects and,y advantages of the invention will become apparent as the present description proceeds.

Referringto the drawing:`

Figure 1 is an elevationalvievv partlyy in section of an electron tube having a cathode sleeve made of thealloy ofthe invention;

Figure 2-is'a fragmentary elevational view partly in Section and appreciably enlarged, of a cathode sleeve incorporating the alloy. of the invention;

Figure 3 shows a graphindicating the thermal conductivity of a nickel-cobalt alloy having varying amounts of.4 cobalt therein; and

Figure 4 depicts graphically the temperature gradients alonga cathode sleeve made of the alloy of the invention and a cathode of the prior art, and as measured by an optical pyrorneter.

Referring to the drawing in more detail, .there is shown inFigure 1 thereohan electron tube having an envelope ;;closed at `one end by a base 11. Within the envelope referred vtoisrnountedan electrode assembly` including a sleeve type cathode 12 energized by a heater 13, and ansanodcy 14. The sleeve and anode are supported by and between insulating yspacer plates 15, 16, which may be.. madeof. mica. The cathode sleeve 12 is connected to a lead-m17 by means ofa tab 18, made for example Several problems are involved for successful operation' ofa cathode of-.this type. One of these problemsconcerns interaction between the coreor base 23- and the emitting` coating 24 thereon. One of these interactions istthexadherence ofsthe coating on the core. Thisadherence is adversely aiected by anv interface produced when silicon. is present in the core alloyinan excessive amount. Applicant has found that whensilicon is present in an amount from 0,04 to 0.10 percent by weight of the alloy,

they objectionable interface formation referred to is` avoided; Applicant hasalso found that a range of carbon contenu in the alloy of from 0.04 to 0.15% vby weight,

with up to .50% `cobalt is satisfactory,A for good results in:,respect, of oxide reduction while preserving` a desired workability of the alloy.

This relatively small amount of carbon in the alloy ispermitted according to the invention becausethe reducingfunction of the .core is aidedfurther by the presence therein of, silicon andmagnesiumgwithin predetermined ranges, as, aforementioned. Applicant hasv found. that magnesium, when present in an amount from, 0.04 to 0.07% by weight of the alloy, does not result to an objectionabledcgree inmagnesium evaporationv and deposit, atcoperating temperatures. of `indirectly. heated cathodes. Therefore, ,accordingto'theinvention. an amount of magnesiinn,withinA this; range- ,isY included in the.k alloy.

In-addition ,to. its. function .cfgV reducinglfvthe@ emitting, mijden magnesium .is ,alsog ,of value infreacting. withr con-Vv taminant sulphur compounds inmthe alloy. As a con- Seqllvsuamacsauese ,Whish susa-ally ,addedtathellox for this purpose may be totally omitted or may be included in a much smaller amount than heretofore found necessary. Specifically, applicant has found that about 0.05% by weight of manganese is satisfactory.

Where .nomanganese ist included in the .alloy, the magnesium is called upon to react-With sulphur contaminants thereby reducingthe anrountof magnesium avail able for,reducingthe.erriittingoxidesin` the coatin This has a relatively small adverse effect on the lifel of the cathode.A However, in critical circumstances,where it is desired to avoid evenuthis smalleifecton the cathode life, manganese may be added to the alloy in the amount of 0.05% by weight of the alloyy to avoid depletion of the magnesium.

Among the characteristics desired in the coreitself are (l) low heat conduction, (2)` good workabilityto permit formation into sleeve shape, and (3) shielding properties to shield the outer surface of the cathodepfromr magneticforces induced by an alternating current energized 'cathode heater.

To secure these characteristics, the core includesnickel Nickel contributes good Workability to the and cobalt. core. Applicant has found that cobalt when included vin the alloy in a predetermined range, adds the characteristics of poor heat conduction and good magnetic shielding of a sleeve formed from the alloy.

While cobalt has the eiect of hardening the alloy, thereby adversely aiecting its workability, applicant has found that cobalt may be included in the alloy up to 50% by weight without prohibitively hardening it infview of the reduced aniountcf carbon aforementioned.

As shown in Figure 3, the measured thermal conductivity vof analloy preponderantly containing nickel and cobalt, varies with the amount of cobalt therein; lt will be noted that when cobalt'is present in an amount from 20 to 50% by weight of the alloy, the alloy is characterized by lowest heat conductivity. This range, it willl be observed, does not exceed the limit of cobaltcontent found tolerable for good workability of the alloy.

ln Figure 4, curve 25 shows the temperature distribution along a cathode having a sleeve core made of the alloy of the invention, and curve 2o shows the temperature distribution along a cathode having a sleeve core of an alloy of the prior art.

The temperature measurements were made` by an optical pyrometer, Therefore, the bare end portions of the cathodes, having brighter surfaces than the intermediate coated portions, are indicated as having higher temperatures than the coated portions, Actually, the coated portion of thecathodes arev hotter than the bare end portions thereof.

lt will benoted that in the case of curve `25, a pro-` nounced bump is evident intermediate the ends of the coated portion. of a cathode including the alloy of the invention. This indicates that the cathode has a much higher temperature at its-intermediate portion.` than at the ends of the coated, portion. This temperature distribution isaconsequence of the relativelyv lowthermal conductivity ofthe cathodesleeve. The temperature dips shownatthe endsof the coated portion of the cathode resultfromheat losses to the insulating spacers engaging.

The. temperature curveA 26` is substantially'flat along vthe coated portion of a cathodesleeve made ofan alloy ofthe` prior art. Thisindicates clearly that-the sleeve-v is characteri'lzedbyvhigh heat conductivity,` which prevents thebuild-upgof a higherrtemperature intermediatethe ends.

ofthe coated portionv than that at said end portions.

They also dissipate heat therefrom, to the spacers and cathode tab. This last mentioned form of heat dissipation is aided appreciably by the high heat conductivity of the sleeve alloy.

The cathode, whose temperature distribution is indicated by curve 25, included a cathode sleeve made of an alloy according to the invention and comprising by weight, 0.05% carbon, 0.04% silicon, 0.05% magnesium, 0.05% manganese, 40% cobalt and the remainder nickel.

On the other hand, the cathode having the temperature distribution of curve 26 included a sleeve comprising 0.5% or less cobalt, 0.20% copper, 0.20% iron, 0.04% magnesium, 0.20% manganese, 0.15 to 0.25% Silicon, 0.008% sulphur as a contaminant, and the remainder nickel.

The presence of cobalt in the alloy also contributes magnetic shielding properties thereto. When a cathode heater is energized by alternating or fluctuating electric current, magnetic fields are generated `about the heater.

In attempting to shield the coating or emissive portion of the cathode from such magnetic fields, applicant has found that the Curiepoint, or the temperature at which with increasing temperature, the core becomes paramagnetic, is signicant. Applicant has found that cobaltnickel alloys of different cobalt content exhibit different Curie points as 1nd1cated 1n the followmg table:

Curie Percent Percent Point Nickel Cobalt Temp. in

Accordingly, for an alloy containing 30% cobalt and the remainder predominately nickel, the Curie point is at 958 K. When the cobalt is increased to 50%, the Curie point is raised to 1123 K. Few indirectly heated cathodes operate below 958 K. or above 1123 K. Therefore, cobalt-nickel alloy cathode sleeves containing from 30 to 50% by weight of cobalt would operate at a temperature below the Curie point of the alloy in most indirectly heated cathodes. Such sleeves, therefore, will not become paramagnetic at operating temperatures.

Of course, by adding more than 50% by weight of cobalt to the alloy, the Curie point of the alloy would occur at a higher temperature than 1123 K. However, since very few indirectly heated cathodes operate at or above this temperature, there is no need from the standpoint of magnetic shielding, to increase the cobalt content beyond the 50% value indicated.

But it is important to note that the lower limit of cobalt content, i. e., 30% by weight of the alloy, cannot be lowered further, without sacrificing good magnetic shielding of the alloy at the operating temperature of a sleeve type cathode having a core made of the alloy. By reducing the cobalt content to a value below 30%, the Curie point would occur at a temperature below 958 K. and therefore possibly below the normal operating temperatures of some types of indirectly heated cathodes.

According to another example, an alloy was prepared containing 0.15 percent carbon, 0.10 percent silicon and 0.07 percent magnesium by weight, as the reducing materials. The alloy also contained 40 percent cobalt and the remainder nickel. No manganese was included in view of the relatively large amount of magnesium employed. The alloy thus formed was more difficult to work than the alloy used in the preceding example. I ascribe this increased difficulty in working to the relatively large carbon content of the alloy. The cathode was operated at a temperature of about 1000 K. Some evidence of a.

silicon interface between the core and the oxide coating thereon, appeared. However, the interface was not thick enough to cause a peeling of the coating from the core. A careful examination of insulators forming part of a device in which the cathode was used, indicated presence of deposits of magnesium thereon but not in sufficient quantity to result in objectionable leakage paths. The core provided good magnetic shielding between the heater and the emitting surface of the cathode. This example clearly indicated, however, that a further increase in any one or all of the carbon, silicon and magnesium in the alloy would result in a defective device.

It is not advisable, in the practice of the invention to employ one reducing material in an amount close to the lower limit of the tolerable range thereof, and another reducing material close to the upper limit of its range according to the invention. This is for the reason that a reduction in the amount of one of the reducing materials will not lessen the disadvantage of a relatively large amount of another of the reducing materials. Thus by reducing the silicon and magnesium content to the lower limits of their ranges, and increasing the carbon to the upper limit of its range, would not reduce the hardening effect of the carbon on the alloy.

It will be apparent from the foregoing that a novel alloy has been provided that includes a novel combination of materials as well as a novel relative amount of materials nowhere taught in the prior art. The materials used and their relative amounts in my novel alloy accomplish the advantageous results of accentuating the desirable features of each of the materials used, particularly the reducing materials, while suppressing the objectionable characteristics thereof. As a consequence of my novel alloy, it is feasible to produce a cathode using my alloy as the core and having an oxide coating thereon including barium oxide, having many advantages over contemporary cathodes. A cathode using my novel alloy is substantially free from incidents of coating peeling, has a high degree of workability for formation into any desirable shape, is characterized by desirable magnetic shielding at operating temperatures, and has a relatively long life of relatively high peak emission.

This is a continuation-in-part of co-pending application Serial No. 193,312 led by applicant October 3l, 1950 now abandoned, for Improvement In Alloy for Cathode Cores, and assigned to the same assignee as the present application.

I claim:

A core for an indirectly heated cathode, said core being made of an alloy consisting by weight of from 0.04 to 0.15% carbon, 0.04 to 0.10% silicon, 0.04 to 0.07% magnesium, cobalt in an amount to preserve the magnetic shielding property of said core at an operating temperature of said cathode in a range from 958 K. to 1123 K., the amount of siad cobalt being substantially rectilinearly related to said temperature from a lower limit of 30% to an upper limit of 50%, and the remainder substantially nickel, whereby said core is adapted to shield an electron emitting coating on the outer surface of said core from magnetic fields, the temperatures in said range being sufficiently low to prevent appreciable loss of said magnesium from said alloy.

References Cited in the le of this patent UNITED STATES PATENTS 2,075,876 VonWedel Apr. 6, 1937 2,192,491 Widell Mar. 5, 1940 2,223,862 Widell Dec. 3, 1940 2,306,290 Widell Dec. 22, 1942 2,478,841 Schmidt Aug. 9, 1949 OTHER REFERENCES Bounds et al.: Proceedings Of The Institute of Radio Engineers, vol. 39, No. 7, July 1951, pages 788-799. 

